Idle seems to have a weird clinking sound to it

[MtoGs21]

I just got this GS500 from 2001 and it's my first two cylinder bike (coming from a 125 ybr) and it sound pretty odd to me at idle

The engine was pretty cold and the rpm were about a constant 900 (might have to adjust it to 1200)

At higher rpms I can't hear that sound anymore
  What could be making that sound? Is it normal? 

video link: https://youtu.be/U7PKseG17LQ

[Bluesmudge]

That just sounds like a bike struggling to idle at 900 rpm. Its supposed to idle at ~1200 rpm when warm (aka after 10 to 15 minutes of riding). You should have to use some choke for the first few minutes to keep the fuel mixture more rich and keep the rpms up. When the bike is cold add choke so its not struggling to idle at 900 rpm.